The present invention relates to a logic system based controller and more specifically a controller that utilizes state machines, logic and/or a microprocessor for an electro-optic system.
Electro-optic systems are known in the art for providing an interface between electronic and optically-based systems. Such electro-optic systems are used in a variety of applications including telecommunications, remote sensing, medical devices, and in other fields as well.
FIG. 1A illustrates a conventional electro-optic system 100 for use with one of the aforementioned systems. The electro-optic system 100 includes a laser 110 and an analog bias controller 120. Typically, an electronic system produces a modulating signal which is combined with a laser bias current Idc. The combined signal is injected into the laser which produces a modulating optical output signal Po, typically an intensity modulated signal in which the light intensity varies as a function of the amplitude of the modulating signal.
In analog systems, the bias current Idc is selected such that it biases the laser at an operating point where the laser output power Po exhibits a linear relationship to the input bias Idc (herein referred to as the laser""s transfer function.). The modulating signal varies the bias point above and below this operating point thereby producing a corresponding change in the intensity of the output power of the laser 110.
The laser 110 will typically includes a monitoring photodiode (not shown) either separate or integrated into the same package with the laser. The monitoring photodiode produces a photodiode current Ipd indicative of the laser output power Po. The photodiode current Ipd is supplied to an analog bias controller 120 which includes circuitry designed to measure the Ipd photodiode current and to return it to a predefined current level.
FIG. 1B illustrates a graph showing three transfer functions of a laser at three operating temperatures. The graph illustrates laser output power along the y-axis and laser bias current Idc along the x-axis. The first trace 152 illustrates the laser""s transfer function at a first operating temperature T1. The second and third traces 154 and 156 similarly illustrate the laser""s transfer function at a second and a third operating temperature, T2 and T3 respectively.
Each of the transfer functions 152, 154 and 156 also include a corresponding threshold operating point Ith1, Ith2, and Ith3 respectively. The threshold operating points indicate the laser bias current level at which the laser produces appreciable output power. As can be seen, the laser""s threshold operating point varies greatly with operating temperature; a change in the operating temperature of the laser shifts the transfer function laterally along the x-axis.
The shift can produce a significant change in the laser output power. For instance, if a laser bias current ldc is selected to bias the laser at operating point Iq1 162 in the center of the linear region of the transfer function, the laser will operate as intended to produce a substantially linearly varying output power when excited by the modulating signal. If the operating temperature of the laser operating temperature changes to T2, the laser output power for the same bias current Iq1, drops significantly as shown by operating point 163. In this case, the linear operating region of the laser and electro-optic system 100 is limited to the linear region below operating point 163. The analog bias controller 120 contains circuits to compensate for change but with a limited degree of accuracy. The analog controller also requires manual adjustments and has limited control capabilities.
Therefore what is needed is a new system and method for controlling a electro-optic system over varying temperatures of operation.
The present invention provides a new system and method for controlling a electro-optic system by which the threshold operating point of the system gain and other parameters can be accurately established and the laser transfer function controlled over a variety of different temperatures. The present invention further provides a system and method for enabling a link characterization process to be performed between two electro-optic systems.
The electro-optic drive controller of the present invention includes a preamplifier output coupled to a laser module system input. The laser module system includes a laser bias input, a laser output, a temperature sensor output, and a monitoring photodiode sensor output. In one embodiment, the present invention provides a method for calibrating the laser module system, including the processes of connecting testing equipment used for laser calibration to the laser output; setting preamplifier gain to nominal; connecting input of preamplifier to ground; turning off the DC bias for laser; canceling the offset voltage of the preamplifier; recording the monitoring photodiode dark current; incrementing the DC bias to the laser; reading PL; computing dPL/dIL; setting the DC bias to threshold; recording PLTH; recording the photodiode current IPT; recording the temperature of laser module; connecting the preamplifier to provide a VREF input signal to the laser module input increment preamplifier gain G1 while reading power output PL with instrument; determining the preamplifier gain GF needed to obtain full scale output of laser power (PLM); determining the corresponding photodiode current for above condition IPM; calculating the effective responsivity Reff of the module containing the laser and the photodiode; and storing the effective module responsivity Reff in memory.
The nature and advantages of the present invention will be better understood with reference to the following drawings and detailed description.